Crossing the Termination Shock

The twin Voyager 1 and 2 spacecraft launched in 1977, are each are now farther away from Earth and the Sun than Pluto is. In fact, they are approaching the boundary region where the Sun’s dominance of the environment ends and interstellar space begins.
Credit: NASA

Using a computer model simulation, Haruichi Washimi, a physicist at UC Riverside, has predicted when the interplanetary spacecraft Voyager 2 will cross the "termination shock," the spherical shell around the solar system that marks where the solar wind slows down to subsonic speed.

According to Washimi’s simulations, the spacecraft is set to cross the termination shock in late 2007-early 2008. To make this forecast, Washimi and his colleagues used data from Voyager 2 and performed a global "magneto-hydrodynamic simulation" – a method that allows for precise and quantitative predictions of geomagnetic disturbances caused by solar activities.

Because Voyager 2’s crossing of the shock is expected to be an abrupt and relatively brief event, scientists are working to ensure that the most is made of the opportunity. With an idea of when the spacecraft will cross the shock, they are better able to maximize coverage of the crossing. As the craft crosses into the outer reaches of the Solar System, it can yield important data about how far the Sun’s influence reaches and how solar wind interacts with galactic space. It’s a unique opportunity to gain information about our Solar System and, ultimately, the features that make it a location capable of supporting habitable planets like Earth.

Earth as seen by the departing Voyager spacecraft: a tiny, pale blue dot.
Credit: NASA

Study results appear in the Dec. 1 issue of The Astrophysical Journal.

"Washimi’s model has predicted the location of a boundary that is approximately 90 times farther from the sun than is the Earth, to within a few percent," said Gary Zank, the director of the Institute of Geophysics and Planetary Physics and one of the coauthors of the research paper. "This is truly remarkable given the enormous complexity of the physics involved, the temporal and spatial scales involved, and the variability of the solar wind conditions."

The solar wind – a stream of charged particles ejected by the sun in all directions – travels at supersonic speeds when it leaves the sun, until it eventually encounters the interstellar medium made up of plasma, neutral gas and dust.

At the termination shock, located at 7-8.5 billion miles from the sun, the solar wind is decelerated to less than the speed of sound. The boundary of the termination shock is not fixed, however, but wobbly, fluctuating in both time and distance from the sun, depending on solar activity.

Voyager 1 took photos of Jupiter and two of its satellites (Io, left, and Europa) on Feb. 13, 1979. This photo was assembled from three black and white negatives by the Image Processing Lab at Jet Propulsion Laboratory. Click image for larger view.
Credit: NASA/JPL

"This is the first time the termination-shock position has been forecast in this way," said Washimi, the lead author of the research paper and a scientist at the Institute of Geophysics and Planetary Physics. "After it crosses this boundary, Voyager 2 will be in the outer heliosphere beyond which lies the interstellar medium and galactic space. Our simulations also show that the spacecraft will cross the termination shock again in the middle of 2008. This will happen because of the back and forth movement of the termination-shock boundary. This means Voyager 2 will experience multiple crossings of the termination shock. These crossings will come to an end after the spacecraft escapes into galactic space."

Voyager 2 was launched Aug. 20, 1977. It visited four planets and their moons in the course of its journey into space. Its sister spacecraft Voyager 1, which was launched Sept. 5, 1977, crossed the termination shock in December 2004 – earlier than Voyager 2 because of a shorter trajectory. The two spacecraft have provided invaluable information about the planets and moons they visited. For instance, they discovered volcanism on Jupiter’s moon Io and returned the first images of Europa’s surface that indicated the moon was internally active. This discovery lead to the hypothesis of a liquid ocean beneath Europa’s crust, and expanded our understanding of how moons of large planets might be able to support unique forms of life.

On the chance that someone is out there, NASA approved the placement of a phonograph record on each of the Voyager spacecraft. The recording, called "Sounds of Earth" fits on a 12-inch, copper disc containing greetings from Earth people in 60 languages, samples of music from different cultures and eras, and natural sounds of surf, wind and thunder, and birds, whales and other animals. The record also contains electronic information that an advanced technological civilization could convert into diagrams, pictures and printed words, including a message from President Carter.
Credit: NASA

Both spacecraft are currently operational, but power sources have degraded and some of the instrumentation no longer works optimally. In the future, the spacecraft will encounter their next milestone in space: the heliopause, which is the boundary where the interstellar medium brings the solar wind to a halt.

Washimi and Zank were joined in the research by UCR’s Qiang Hu; Takashi Tanaka of Kyushu University, Japan; and Kazuoki Munakata of Shinshu University, Japan. The research was funded by grants from the National Science Foundation and the National Aeronautics and Space Administration.